<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    /* 子元素作为父元素的第一个元素，它给margin-top，没用
       它的元素向下移动 */
    .parent{
        width: 200px;
        height: 200px;
        background: salmon;
    }
    .child{
        margin-top: 50px;
        width: 100px;
        height: 100px;
        background: skyblue;
    }
    /*  如何解决  */
    .row:before{
        content: "";
        display: table;
    }
    
    </style>
</head>
<body>
    <div class="parent row">
      
        <div class="child">

        </div>
    </div>
</body>
</html>